2 But “Jesus answered and said unto him, What I do you know not now, but you shall know hereafter.” And not even yet, terrified as he was by the sublimity of the Lord's action, does he allow it to be done, while ignorant of its purpose; but is unwilling to see, unable to endure, that Christ should thus humble Himself to his very feet. “You shall never,” he says, “wash my feet.” What is this “never” [in æternum]? I will never endure, never suffer, never permit it: that is, a thing is not done “in æternum” which is never done. Then the Saviour, to terrify His reluctant patient with the danger of his own salvation, says, “If I wash you not, you shall have no part with me.” He speaks in this way, “If I wash you not,” when He was referring only to his feet; just as it is customary to say, You are trampling on me, when it is only the foot that is trampled on. And now the other, in a perturbation of love and fear, and more frightened at the thought that Christ should be withheld from him, than even to see Him humbled at his feet, exclaims, “Lord, not my feet only, but also my hands and my head.” Since this, indeed, is Your threat, that my bodily members must be washed by You, not only do I no longer withhold the lowest, but I lay the foremost also at Your disposal. Deny me not having a part with You, and I deny You not any part of my body to be washed.
3. “Jesus says to him, He that is washed needs not save to wash his feet, but is clean every whit.” Some one perhaps may be aroused at this, and say: Nay, but if he is every whit clean, what need has He even to wash his feet? But the Lord knew what He was saying, even though our weakness reach not into His secret purposes. Nevertheless, so far as He is pleased to instruct and teach us out of His law, up to the little measure of my apprehension, I would also, with His help, make some answer bearing on the depths of this question: and, first of all, I shall have no difficulty in showing that there is no self-contradiction in the manner of expression. For who may not say, as here, with the greatest propriety, He is all clean, except his feet?— although he would speak with greater elegance were he to say, He is all clean, save his feet; which is equivalent in meaning. Thus, then, does the Lord say, “He needs not save to wash his feet, but is all clean.” All, that is, except, or save his feet, which he still needs to wash.
Source: Tractates on the Gospel of John (New Advent)